1. Field of the Invention
The present invention relates to fabric stretchers and more particularly to an apparatus for tensioning and re-tensioning screen fabric for use in screen printing processing or the like.
2. Description of the Prior Art
Stretching frames are used for a wide variety of applications. One prevalent use for such frames is in the field of graphic arts. In the medium of oil painting, the canvass must be stretched into a taught plane so the ink may be applied onto a firm and stationary surface. Frequently the frame used for such art is made of wood and the canvass is attached to one side and then pulled across to the opposite side where it is anchored. Such frames may be fashioned so that a wedge may be driven into each of the corners of the frame to provide tension for the tightening of the canvas.
Printing screens may be used for transfer of inks for the creation of an art work or in the printing industry. Screen mesh as used in the screen print industry is manufactured from polyester and is typically supported and tensioned between opposing sides of a four sided frame. The mesh, when uniformly stretched in a taught plane, provides the foundation for a stencil. Printing ink is then forced through the open areas of the stencil by means of a blade or squeegee creating an image in the shape of the stencil on the work piece positioned beneath the screen. To achieve optimum results, several factors must be considered. The screen must be maintained at the recommended working tension throughout the print run so the pattern is transferred with accuracy. Tension variation must be kept to a minimum. Off contact must be kept to a minimum.
During the printing stroke, the squeegee blade deflects the fabric downwards forcing it into direct contact with the work piece during its passage across the screen. This deflection causes the fabric to be stretched from its undisturbed condition. The constant cycle of stretching and snapping back accelerates the deterioration of both the screen and the stencil, especially if the panel is skewed during installation. Proper tensioning decreases off contact, improves image quality and provides longer stencil and image life. Additionally, as the screen mesh undergoes these distorting forces during the print stroke, the stencil in the screen may also be distorted, especially if the contact between the screen and the fabric is redundant. This can result in a slightly distorted image on the work piece.
Typical tubular alloy frames used to stretch the screen mesh utilize tubes positioned on all four sides of the frame. The screen is pulled over these tubes and locked in place without the necessary squaring means. In doing so, the frame may become slightly distorted and this distortion can cause the off contact distance between the screen and the work piece to vary across the print area with a consequent reduction in print quality.
The essence of re-tensioning is to maintain a specific tension value in newtons. The majority of screen frames currently available do not allow the mesh and stencil to be easily removed and reinstalled on the frame. Consequently screens must be stored mounted on their frames between print runs. Removal of the mesh and accurate reinstallation for later use represents an improvement over the prior art. Applicant's U.S. Pat. No. 6,739,079 addressed the aforesaid problems and this application represents a novel contribution to those teachings and the screen printing industry.